However, the first report of habitat-adapted symbiosis 20 years ago revealed that plant adaptation in high-stress habitats ( Redman et al., 2002b) can occur on an intergenomic level via symbiosis with Class 2 fungal endophytes ( Rodriguez et al., 2009 Goh et al., 2013). Since these genes are found in the plant's nuclear genome, their expression is presumed to be responsible for the adaptation. The biochemical basis for this adaptation is thought to involve particular proteins involved in sulfur and selenium uptake and transport, coded by genes such as SHST1, SHST2, and SHST3 ( Terry et al., 2000). For example, some plants are adapted to the presence of selenium enabling them to grow in soils with high concentrations of the element that limit the distribution of the plant's competitors ( El Mehdawi et al., 2011a, b). The cellular and mechanistic processes responsible for the adaptive potential and phenotypic plasticity of plants are still largely undefined but thought to involve processes exclusive to the plant's genome and considered as the primary factor responsible for plant distribution patterns and biogeography ( Chevin et al., 2010 Matesanz et al., 2010 Nicotra et al., 2010 Zhang et al., 2013 Gratani, 2014 Zhou et al., 2019 Liu et al., 2020 Monforte, 2020 Abady et al., 2021 KlupczyĆska and Ratajczak, 2021 Stotz et al., 2021 Syngelaki et al., 2021 Yang et al., 2021 Yu et al., 2021 Wang et al., 2022). The geographic pattern and distribution of plants across complex habitats have been extensively studied and well-documented ( Martyn, 1729 Bradshaw, 1965 Crisci, 2001). Our studies indicate that Class 2 fungal endophytes can provide a symbiotic mechanism for niche expansion and phenotypic plasticity across environmental gradients. Conversely, the inability of an endophyte to confer stress tolerance resulted in a decrease of in planta fungal abundance. The ability of an endophyte to confer appropriate stress tolerance resulted in a significant increase of in planta fungal abundance. Modulation of endophyte abundance occurred in planta based on the ability of the symbiont to confer tolerance to the stress imposed on plants. In contrast, when dunegrass and Grindelia integrifolia (gumweed) were found growing in low salinity, but high drought habitats, these plant species had their own unique dominant endophyte association regardless of geographic proximity and conferred drought but not high salt stress tolerance. A reciprocal transplant study along a salt gradient demonstrated that Leymus mollis (dunegrass) required endophytes indigenous to each microhabitat for optimal fitness and/or survival. At the microhabitat interfaces where the gradation of salinity varied, the plants were colonized by endophytes from both microhabitats. Endophyte analysis of three native ( Leymus mollis, Distichlis spicata, and Salicornia pacifica) and one invasive ( Spartina anglica) plant growing across adjacent microhabitats in the San Juan Archipelago altered associations with Class 2 fungal endophytes in response to soil salinity levels. Here, we present studies showing that plants can grow across complex habitat gradients by modulating symbiotic associations with Class 2 fungal endophytes. Modern evolutionary theory and population genetics posit that adaptation and habitat expansion of plants result from processes exclusive to their genomes. Adaptive Symbiotic Technologies, Seattle, WA, United States.
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